I have a polygon Feature Layer in the ArcGIS Online Portal that has labels. The labels will display fine if nearly the full feature is within the extent of the map view. However if I zoom in the label will disappear. If I pan to roughly the center of the feature, I will see the label, but anywhere other than the center I will not.
This can be demonstrated by using the states map service.
If you create a Feature Layer using this service, label it using the name at all scales, then add it to a .Net Runtime (100.6) Map, once you zoom in to around the City level the labels disappear. The same thing happens in the AGOL Map Viewer, and various Map Templates.
How can polygon labels be displayed so they show at all scales, if you're not looking at the centroid of the polygon?
Unfortunately, I believe this is a known bug that still exists in the 100.6 release.
BUG-000114932 When the map view is zoomed or panned, labels do not reposition to move in view as they do in ArcGIS Pro.
I believe Kyle is correct - it sounds like you're seeing the referenced issue. Some additional info - we had a candidate fix in place for the v100.6 release but we were not completely happy with the label placement behavior around the edges of the geoview and had to revert the change. The fix is currently in our internal development builds and will undergo testing in preparation for the next release (v100.7).